Non-aqueous electrolyte secondary batteries have a high operating voltage and high energy density. In recent years, non-aqueous electrolyte secondary batteries have been commercialized as power sources for driving portable electronic equipment such as cell phones, notebook computers and video camcorders, and the demand thereof is rapidly growing. Particularly, production of lithium ion secondary batteries is steadily increasing, dominating the field of small secondary batteries.
In order to achieve a voltage as high as 4 V, a lithium ion secondary battery usually employs a lithium-containing composite oxide as a positive electrode active material. Typically used are LiCoO2 and LiNiO2 having a hexagonal crystal structure, and LiMn2O4 having a spinel structure. Among them, dominant are LiCoO2 and improved types of LiCoO2 because they can offer a high operating voltage and high energy density.
The lithium ion secondary battery usually employs, as a negative electrode material, a carbon material capable of absorbing and desorbing lithium ions. In order to achieve a flat discharge potential and high capacity density, graphite materials are dominantly used.
Only recently has the movement begun to utilize non-aqueous electrolyte secondary batteries in higher capacity battery applications including electric power storage systems and electric vehicles, in addition to in small consumer applications. In the field of hybrid electric vehicles (HEVs), vehicles equipped with nickel-metal hydride storage batteries have already been manufactured on a mass production basis and commercially available. In the wake of this stream, the development of vehicles equipped with, instead of nickel-metal hydride storage batteries, non-aqueous electrolyte secondary batteries has also progressed rapidly.
In the future, the widespread use of fuel cell vehicles is expected. Non-aqueous electrolyte secondary batteries are now viewed as a promising candidate as a secondary battery for assisting a fuel cell that is capable of offering long life and high input/output power performance.
The levels of performance required for non-aqueous electrolyte secondary batteries for HEV and fuel cell vehicle applications differ greatly from those for small consumer applications. Because the batteries for HEV and fuel cell vehicle applications need to instantly provide power assistance to an engine or to contribute the regeneration with a limited capacity, they are required to have higher input/output power performance. For this reason, a higher priority is placed on the achievement of higher input/output power performance, rather than the achievement of higher energy density. Further, for achieving higher input/output power performance, it is necessary to minimize the internal resistance of a battery. Under the circumstances, in order to achieve a significant improvement in input/output power performance, attempts have been made not only to develop or select suitable active materials or electrolytes, but also to improve the current collecting structure of electrodes, to reduce resistances of battery constituting components and to make electrodes longer and thinner so as to increase the reaction area of electrodes.
Non-aqueous electrolyte secondary batteries of high input/output power performance are also expected to serve as a power source for high power applications such as a power source for driving power tools, as well as to outperform existing nickel-cadmium storage batteries and nickel-metal hydride storage batteries.
In the designing process of a non-aqueous electrolyte secondary battery of high input/output power performance, as described above, important factors are the improvement of the current collecting structure and the reduction of resistances of battery constituting components. The improvement or selection of electrode active materials is also an important factor. Particularly in a low temperature environment, the latter factor largely affects the input/output power characteristics. Especially, the capability of carbon material in the negative electrode to absorb and desorb lithium ions is of significant importance. The improvement of this capability greatly contributes to the achievement of a battery of high input/output power performance.
As described earlier, the combination of the positive electrode and the negative electrode commonly employed in small consumer applications is not an optimal combination for yielding a high power output non-aqueous electrolyte secondary battery. In short, the combination of a positive electrode including LiCoO2 as an active material and a negative electrode including graphite is not optimal.
Under the circumstances, the combination of a positive electrode including a lithium-containing composite oxide other than LiCoO2 and a negative electrode including a low crystalline carbon material is considered as promising. As the low crystalline carbon material, the use of, for example, a non-graphitizable carbon material (hard carbon) is being examined.
Non-aqueous electrolyte secondary batteries of high input/output power performance are rarely charged from a discharged state to a fully charged state with a single charge. It is also rare that they are continuously discharged from a fully charged state. Usually, pulse charge/discharge cycles are repeated every specified seconds around a state-of-charge (SOC) of 50 to 60%. Accordingly, unlike batteries for small consumer applications, non-aqueous electrolyte secondary batteries of high input/output power performance are required to achieve a long cycle life when repeatedly charged and discharged with a pulse current. Also, non-aqueous electrolyte secondary batteries of high input/output power performance are required to have a capability to repeatedly charge and discharge regardless of the magnitude of pulse currents.
Hitherto, various proposals have been made for negative electrode materials of non-aqueous electrolyte secondary batteries. Examples thereof include a graphite material which is dominantly used for small consumer applications (Japanese Laid-Open Patent Publication No. 2000-260479), a low crystalline non-graphitizable carbon material (Japanese Laid-Open Patent Publication No. 2000-200624) and a pseudo graphite material having a controlled graphitization degree (Japanese Laid-Open Patent Publication No. 2000-260480). Carbon materials having specific physical properties specified using diffraction patterns obtained by wide-angle X-ray diffraction are also proposed. More specifically, Japanese Laid-Open Patent Publication No. Hei 6-275321 proposes a material having an intensity ratio I(101)/I(100) of a peak intensity I(101) attributed to a (101) plane to a peak intensity I(100) attributed to a (100) plane that satisfies 0.7≦I(101)/I(100)≦2.2. The publication particularly recommends a carbon material having an intensity ratio I(101)/I(100) of not less than 0.8 or not less than 1.0.
However, when an electrode including a conventional carbon material is charged and discharged with a pulse current (i.e., when pulse charge/discharge is performed) for a long period of time as described above, its capability to charge and discharge decreases, which induces capacity degradation as well as the increase of internal resistance. Such a battery is not suitable for long-term use and not practical at all because, when used in an HEV, for example, it cannot provide sufficient power assistance or cannot sufficiently contribute to the regeneration.
A graphite material of Japanese Laid-Open Patent Publication No. 2000-260479, for example, has a layered structure in which hexagonal crystals are regularly arranged in the c-axis direction. During charge, lithium ions are intercalated between the layers to widen a spacing between the layers, and the graphite material expands. The stress accompanied by such expansion gradually increases during the repetition of pulse charge/discharge at a large current. Thereby, the charge acceptability of the graphite material gradually decreases, resulting in a short cycle life. In other words, the graphite material is not suitable as a negative electrode material for a battery to be subjected to repeated pulse charge/discharge.
Moreover, graphite powders, although it depends on the particle shape or the like, tend to be oriented in the c-axis direction. Accordingly, the site where lithium ions are selectively absorbed and desorbed is limited, and thus they are not suitable for a large current charge.
As for a non-graphitizable carbon material of Japanese Laid-Open Patent Publication No. 2000-200624, its charge/discharge reaction mechanism differs from the reaction mechanism of the graphite material, which means the intercalation of lithium ions between layers hardly occurs. Because most lithium ions are intercalated into the pores of the carbon material, the stress caused by expansion or contraction during charge/discharge is small. Further, because the non-graphitizable carbon material has a low degree of orientation, the sites where lithium ions are absorbed and desorbed are randomly arranged therein. For this reason, it can be said that the non-graphitizable carbon material is suitable for a large current charge. The non-graphitizable carbon material, however, has lower electroconductivity than graphite materials. As such, the internal resistance increases during discharge in which lithium ions leave. This tendency becomes remarkable particularly when a large current discharge is repeated.
A pseudo graphite of Japanese Laid-Open Patent Publication No. 2000-260480 has a crystallite thickness Lc in the c-axis direction of not less than 60 nm and less than 100 nm. In other words, it is a carbon material having a relatively high degree of graphitization. Accordingly, similar to the case of graphite materials, the use of pseudo graphite is likely to decrease charge acceptability.
Further, a battery proposed by Japanese Laid-Open Patent Publication No. Hei 6-275321 has a negative electrode plate with a thickness of 180 μm and a positive electrode plate with a thickness of 270 μm. Such a battery having thick electrode plates is not designed to offer high input/output power performance.
Carbon materials having an intensity ratio I(101)/I(100) of not less than 0.8 or not less than 1.0 are usually classified as a graphite material having a developed layered structure formed of hexagonal crystals. In such carbon materials, layers are slightly displaced or twisted as compared to natural graphite having a structure similar to monocrystal. Accordingly, almost similar to the case of the graphite material, the use of such carbon material is likely to decrease charge acceptability owing to its high crystallinity.
As discussed above, it is difficult for a non-aqueous electrolyte secondary battery including a conventional carbon material in the negative electrode to achieve a long cycle life when subjected to repeated charge/discharge cycles at a pulse current. Particularly in a low temperature environment, it is difficult to achieve high input/output power performance during pulse charge or pulse discharge at a large current.